Network access is a prime suspect for arbitrary long delays like that. Work locally out of a Git or SVN repo and commit as needed. This includes avoiding symlinks to network locations, or sync/mirror services that are set way too aggressively (maybe see if there's a setting to sync only every so often?).
Having too many libraries installed can be troublesome, or other resources (SVN? Vault stuff? Dunno, never used a private vault).
Large (C sheet+) schematics can be slow, give or take computer performance; have maintained a few older designs like that. That's not a delay thing, just a framerate thing. A or B (or A4/A3) sheets are easier to read anyway. What are you drawing schematics for if not to read!..
I've got a 2010 laptop with XP, running AD15 (last version supporting XP), it's fine. NVIDIA graphics is a must; Intel or other onboard graphics may be okay these days but YMMV. You're better off following Altium recommendations. Also if you've got one of those power management settings tools, make sure it's set to use the GPU with Altium. Mostly a framerate thing again, though PCB stuff may just not show if it's really bad.
And moving things, can have slowness from compiling, online DRC, building planes, or populating tables. I don't know how to avoid compiling (I also don't use the latest version that does it constantly), but the rest can be disabled or mitigated. Planes in particular are irritating as the algorithm is really, really bad; I recommend just not using plane layers at all, use polygons on signal layers instead (and repour them as needed, not automatically). Tables include PCB List panel and drill table. If you're using PCB List, set it to selected only, so it's not querying 10k+ objects every time anything changes... (The same doesn't work on SCH I think, and I tend to leave that one disabled.)
Tim